Gene localization is done by in situ hybridization of radioactive RNA to human chromosomes. Our previous work has located about half of the 5S rRNA genes on the long arm of chromosome one. Current research is concentrating on mapping the 5S rRNA genes in a linear order on chromosome 1. New and improved technical advances may permit the detection of gene sequences at lower levels of DNA repetition than were previously possible. We will attempt to map the genes coding for the histones. There is an interesting correlation between the human DNA satellites I, II, III and IV with the chromosomes which have the ribosomal precursor genes. Interphase nuclei with genetic variant "C bands" are being examined to understand the role of these DNA satellites in positioning the ribosomal genes during interphase when the nucleolus is making ribosomes. The 5S genes and histone mRNA are being examined at interphase in further detail. Before attempting to map specific tRNA species on human chromosomes, we will isolate and purify tRNA in a large scale, selecting a few major species for high purification and RNA:DNA hybridization on nitrocellulose filters. Drosophila is serving as our model system before human chromosomes are done. BIBLIOGRAPHIC REFERENCES: Steffensen, D.M., W. Prensky, D. Mutton and J.L. Hamerton (1975) Cytogenet. Cell Genet. 14 434-438 (Human Gene Mapping 2, Rotterdam Conference, (1974)). Peacock, W.J. and D.M. Steffensen (1975). Mapping the highly repeated sequences, 1705 of Drosophila melanogaster. J. Cell Biol. 67, 328a (abstract).